1. Field of the Invention
The present invention relates to an optical filtering module and various optical devices using the filtering module. More particularly, the present invention relates to an optical filtering module including an optical filter, a first optical system for guiding light entering the optical filter, and a second optical system for guiding light exiting the optical filter, as well as various optical devices utilizing such optical filtering module, such as an optical multi/demultiplexer or an optical equalizer.
2. Description of the Related Art
In the optics industry field, an optical filtering module is employed comprising an optical filter for selectively transmitting, attenuating or reflecting light, and an optical system for guiding light to the optical filter or guiding light exiting the filter. Among such optical filtering modules, an optical multi/demultiplexer capable of multiplexing light having different wavelength signals or demultiplexing light into individual different wavelength light signals is well-known, as is an optical equalizer for equalizing light having an uneven intensity/wavelength property into light having a constant intensity/wavelength property.
FIG. 20 shows an optical multi/demultiplexer 90 taken as an example for an optical filtering module. The optical multi/demultiplexer 90 comprises a first collimator 100 having two optical fibers 101, 102, a second collimator 110 having a single optical fiber 111, and a wavelength selection filter 95 disposed between the collimators 100, 110.
The first collimator 100 and the second collimator 110 comprise respective fiber holders 104, 114 made of ceramics for holding the optical fibers therein. The fiber holder 104 of the first collimator 100 is formed with two through holes having a circular cross section and extending in a longitudinal direction. The fiber holder 114 of the second collimator 110 is formed with one through hole having a circular cross section and extending in a longitudinal direction. The optical fibers are held by the holders by being inserted into the through holes and bonded by an adhesive.
The first and second collimators 100, 110 comprise respective rod lenses 103, 112 for collimating and converging light. These rod lenses 103, 112, together with the optical fiber holders 104, 114, are held by respective inner holders 105, 115, which are in turn held by an outer holder 120. These inner and outer holders 105, 115, 120 are made of a metal such as stainless steel, or surface-treated with gold plating. The inner holders 105, 115 are bonded to the outer holder 120 by means such as soldering or YAG welding.
Graded index lenses with a pitch of 0.25 are normally used as the rod lenses 103, 112. The rod lenses 103, 112 and optical fibers are mated at their respective oblique surfaces inclined by 8 degrees and polished and are bonded together by an adhesive 107 coated around the circumference of the mated surfaces.
When demultiplexing light using the optical multi/demultiplexer 90, inputted light transferred through one of optical fibers of the first collimator 100 is collimated by the rod lens 103 and reaches the wavelength selection filter 95. The light with a certain wavelength range is reflected by the wavelength selection filter 95 to pass through the rod lens 103, and exits through the other optical fiber of the first collimator 100. On the other hand, the light transmitted through the wavelength selection filter 95 is then converged by the rod lens 112 of the second collimator 110 and exits through the optical fiber 111.
When multiplexing light using the optical multi/demultiplexer 90, light transferred through the optical fiber 111 is multiplexed with light introduced through one of the optical fibers of the first collimator 100 and exits through the other optical fiber of the first collimator 100.
In the above-described conventional multi/demultiplexer, the outer holder is made of an opaque material such as metal, which creates manufacturing problems because it is difficult to see the inside of the holder. That is, component elements may collide with the holder or other elements and broken when they are inserted into the holder or being fixed to other elements. Also, aligning operation for the element is difficult to reduce the accuracy.
In addition, in the conventional manufacturing process, the inner holder is often fixed by soldering or YAG welding, which requires a surface metalization treatment of the inner holder, resulting in complication of the whole process. Also, the optical elements are heated in those processes and subjected to expansion and shrinkage thereof, resulting in deteriorating the positioning accuracy.
Also, the outer holder made of metal, due to its high thermal expansion coefficient, is likely to expand or shrink depending on the environmental condition, to cause displacement of component elements to decrease efficiency of the device.
The object of the present invention is, therefore, to provide an optical filtering module capable of facilitating an easy alignment of the component elements when manufacturing, and providing a stable tuning characteristics by avoiding displacement and, consequently, a long term use reliability, as well as various optical devices using such an optical filtering module.